1. Field of Invention
The present invention relates to spatial light modulators, and, more particularly, to spatial light modulators with electronically addressable deflectable elements attached to an optically transmissive substrate.
2. Description of Prior Art
Spatial light modulators (SLMs) are transducers that modulate an incident beam of light in a spatial pattern that corresponds to an optical or electrical input. The incident light beam may be modulated in phase, intensity, polarization, or direction. This modulation may be accomplished through the use of a variety of materials exhibiting magneto-optic, electro-optic, or elastic properties. SLMs have many applications, including display systems, optical information processing, optical data storage, and printing.
A common technology for an SLM cell is to use a liquid crystal material sandwiched between two electrodes, at least one of the electrodes being transparent. By applying a voltage between the electrodes, the orientation of the molecules in the liquid crystal layer changes, which alters the optical properties of the layer, in particular the polarization of light traveling through the layer. Thus, the liquid crystal layer in combination with one or more polarizing filters can be used to create an amplitude modulator (light valve). However, such liquid crystal based devices have several disadvantages for SLM applications. First, much of the light is absorbed in the polarizing filters, reducing optical efficiency. In addition, the devices have limited contrast ratio, (the ratio of the intensities of the pixel when on and the pixel when off), and the response time of the most widely used liquid crystals is very slow (several milliseconds). Liquid crystals also have poor performance outside a fairly narrow temperature range. For these reasons and others, mechanical SLMs, which use moving structures to deflect light, have been pursued.
An early mechanical SLM designed for use in a projection display system is described by Nathanson, U.S. Pat. No. 3,746,911. The individual pixels of the SLM are addressed via a scanning electron beam as in a conventional direct-view cathode ray tube (CRT). Instead of exciting a phosphor, the electron beam charges deflectable reflective elements arrayed on a quartz faceplate. Elements that are charged bend towards the faceplate due to electrostatic forces. Bent and unbent elements reflect parallel incident light beams in different directions. Light reflected from unbent elements is blocked with a set of Schlieren stops, while light from bent elements is allowed to pass through projection optics and form an image on a screen.
Another electron-beam-addressed SLM is the Eidophor, described in E. Baumann, xe2x80x9cThe Fischer large-screen projection system (Eidophor)xe2x80x9d 20 J. SMPTE 351 (1953). In this system, the active optical element is an oil film, which is periodically dimpled by the electron beam so as to diffract incident light. A disadvantage of the Eidophor system is that the oil film is polymerized by constant electron bombardment and oil vapors result in a short cathode lifetime. A disadvantage of both of these systems is their use of bulky and expensive vacuum tubes.
A spatial light modulator in which movable elements are addressed via electrical circuitry on a silicon substrate is described in K. Peterson, xe2x80x9cMicromechanical Light Modulator Array Fabricated on Siliconxe2x80x9d 31 Appl. Phys. Let. 521 (1977). This SLM contains a 16 by 1 array of cantilever mirrors above a silicon substrate. The mirrors are made of silicon dioxide and have a reflective metal coating. The space below the mirrors is created by etching away silicon via a KOH etch. The mirrors are deflected by electrostatic attraction: a voltage bias is applied between the reflective elements and the substrate and generates an electrostatic force. A similar spatial light modulator is the two-dimensional array described by Hartstein and Peterson, U.S. Pat. No. 4,229,732. Although the switching voltage of this SLM is lowered by connecting the deflectable mirror elements at only one corner, the device has low efficiency due to the small optically active area (as a fraction of the entire device area). In addition, diffraction from the addressing circuitry lowers the contrast ratio of the display.
A silicon-based micro-mechanical SLM in which a large fraction of the device is optically active is the Digital Mirror Device (DMD), developed by Texas Instruments and described by Hornbeck, U.S. Pat. No. 5,216,537 and its references. The most recent implementations include a first aluminum plate suspended via torsion hinges above addressing electrodes. A second aluminum plate is built on top of the first and acts as a mirror. The double plate aluminum structure is required to provide an approximately flat mirror surface that covers the underlying circuitry and hinge mechanism, which is essential in order to achieve an acceptable contrast ratio. The entire structure is made from aluminum alloys-the plates, torsion hinges and special xe2x80x9clanding tipsxe2x80x9d each have independently optimized compositions. Aluminum can be deposited at low temperatures, avoiding damage to the underlying CMOS addressing circuitry during manufacture. Aluminum has the disadvantage, however, of being susceptible to fatigue and plastic deformation, which can lead to long-term reliability problems and cell xe2x80x9cmemoryxe2x80x9d, where the rest position begins to tilt towards its most frequently occupied position. Additional disadvantages of the DMD include: 1) A large dimple (caused by the mirror support post) is present at the center of the mirror in current designs which causes scattering of the incident light and reduces optical efficiency. 2) The entire DMD structure is released via plasma etching of a polymer sacrificial layer. This manufacturing process is problematic, in that it (a) requires large gaps between mirrors in order for the plasma etch release to be effective, and (b) pixel failures are created during the release process, which is not sufficiently gentle on the delicate micromirror structures. Due to the complex structure and process difficulties, commercialization of the DMD has proceeded slowly.
Another SLM fabricated on a flat substrate is the Grating Light Valve (GLV) described by Bloom, et. al., U.S. Pat. No. 5,311,360. As described in the ""360 patent, the GLV""s deflectable mechanical elements are reflective flat beams or ribbons. Light reflects from both the ribbons and the substrate. If the distance between the surface of the reflective ribbons and the reflective substrate is one-half of a wavelength, light reflected from the two surfaces adds constructively and the device acts like a mirror. If this distance is one-quarter of a wavelength, light directly reflected from the two surfaces will interfere destructively and the device will act as a diffraction grating, sending light into diffracted orders. A favored approach is to make the device from ceramic films of high mechanical quality, such as LPCVD (low pressure chemical vapor deposition) silicon nitride.
Even though addressing circuitry cannot be placed below such films, an inherent electromechanical bistability can be used to implement a xe2x80x9cpassivexe2x80x9d addressing scheme (Raj Apte, Grating Light Valves for High Resolution Displays, Stanford University Ph.D. thesis, June 1994). The bistability exists because the mechanical force required for deflection is roughly linear, whereas the electrostatic force obeys an inverse square law. As a voltage bias is applied, the ribbons deflect. When the ribbons are deflected past a certain point, the restoring mechanical force can no longer balance the electrostatic force and the ribbons snap to the substrate. The voltage must be lowered substantially below the snapping voltage in order for the ribbons to return to their undeflected position. This latching action allows driver circuitry to be placed off-chip or only at the periphery, and addressing circuitry does not need to occupy the optically active part of the array. In practice, this approach is difficult to implement: when the ribbon comes into contact with the substrate, which is at a different potential, charge can be injected into the insulating ceramic ribbon material, shifting the switching voltages and making passive addressing impossible. Film non-uniformity across the device can also shift the switching voltages significantly. Another problem with the GLV technology is sticking: since the underside of the deflected ribbons contacts the substrate with a large surface area, the ribbons tend to stick to the substrate. Films comprising the structure can be roughened, but this results in undesirable optical scattering, reducing the contrast ratio of the device.
Micro-mechanical mirror-based SLMs have an advantage over diffraction-based SLMs because they reflect incident light at only one angle, which can be quite large. This simplifies the design of the optical system in which the modulated light may pass through the center of the imaging lens, while maintaining high efficiency. This results in an image with fewer aberrations and lowers manufacturing cost.
The need therefore is for a spatial light modulator with a high contrast ratio, high efficiency, high speed, which is easy to fabricate, and whose moving elements are made of reliable mechanical materials.
Briefly, in accordance with an embodiment of this invention, a spatial light modulator comprises an optically transmissive substrate and a circuit substrate. One or more reflective deflectable elements are attached to the lower surface of the optically transmissive substrate. This optically transmissive substrate is held above, and spaced apart from, a circuit substrate containing addressing circuitry capable of selective activation of each reflective deflectable element.
In operation, individual reflective elements are selectively deflected and serve to spatially modulate light that is incident to, and then reflected back through, the optically transmissive substrate.
In one embodiment of this invention, the spatial light modulator comprises an array of pixels. Each pixel comprises a single deflectable rigid mirror and a torsion hinge, which attaches the mirror to an upper, optically transmissive substrate. The optically transmissive substrate is held above a silicon substrate, on which is formed an array of electrodes. In one embodiment, an aperture layer is built into the optically transmissive substrate to block light from reaching the electrodes or the mirror support structure (hinges and attachments). Individual mirrors are selectively deflected electrostatically by applying a voltage bias between individual mirrors and their corresponding electrodes.
In accordance with an embodiment of this invention, a process for fabricating the spatial light modulator is provided. A sacrificial layer is deposited on a substrate. A hole is etched through the sacrificial layer, the hole allowing for attachment of subsequent layers to the optically transmissive substrate. A reflective layer is deposited on the sacrificial layer, and is patterned to define one or more reflective deflectable elements. The reflective layer is connected to the sacrificial layer through the hole. The sacrificial layer is removed so that the reflective elements are free and may deflect. Addressing circuitry and electrodes are formed on a circuit substrate. The substrate and circuit substrate are aligned and joined such that the reflective elements may be selectively actuated by the addressing circuitry and electrodes. The two substrates may be joined, for example, by epoxy around the periphery of the substrates.
In accordance with an embodiment of this invention, a process includes asserting a bias voltage between the reflective deflectable element and the addressing circuitry. The bias voltage may be changed during device operation.
The electrical addressing circuitry on the silicon substrate may be fabricated using standard CMOS technology, and resembles a low-density memory array.
Since the two substrates are joined together only after they are individually fabricated, the fabrication processes for each substrate are decoupled. As there is no concern for CMOS compatibility during the manufacturing of the top substrate, an advantage of the spatial light modulator of this invention is that the mechanically deflectable reflective elements can be made from materials chosen only for their excellent mechanical properties, such as LPCVD-deposited silicon nitride, silicon oxide, amorphous silicon and poly-silicon. Since these films are deposited at high temperatures, they are not normally compatible with CMOS processes, because the latter use aluminum interconnects which would melt at these higher temperatures.
A further advantage of this spatial light modulator is that after the two substrates are bonded together, the moving parts may be fully encapsulated. This provides an excellent method of packaging and leads to high device robustness.
The spatial light modulator of this invention has the further advantage that it is inexpensive and straightforward to construct. It is composed of two substrates: one which may be made using standard CMOS techniques, and a second optically transmissive substrate containing the deflectable reflective elements, which is very simple to fabricate.
Yet another advantage of this spatial light modulator is that a light blocking aperture layer, as well as other planar optics (e.g. color filters, reflectivity enhancement coatings, micro-lenses) can be incorporated into the optically transmissive substrate. This can improve the contrast ratio and increase the effective light deflection angle, and reduce the cost of free-space optics at the systems level.
Yet another advantage of this spatial light modulator is that the motion limiting structures can also be made of high-temperature materials which are hard and have long lifetimes. Because of their hardness and geometry, the motion limiting structures have a small contact area during operation, which greatly reduces sticking forces between the structures and the substrate. Also, the motion limiting structures are at the same electrical potential as the substrate with which they come into contact which prevents sticking via welding and charge injection. These were problems encountered with early versions of the DMD and the GLV.
Yet another advantage of this spatial light modulator is that the high-temperature processing of the optically transmissive substrate allows for the deposition of dielectric films with alternating high-low indices of refraction onto the deflectable reflective elements, which enhance their reflectivity.
These and other advantages will become apparent to those skilled in the art after consideration of the ensuing drawings and detailed description.